The atmospheric leaching of copper ores can be carried out using a variety of contactor types. Copper ores can be leached on heaps or dumps, in vats or in stirred tanks. The choice of the type of contactor will vary with the type and grade of the ore and its leach characteristics as well as local economic, environmental, topological and hydrogeological character.
The leaching of copper minerals generally proceeds by direct acid attack for simple oxides, carbonates and silicates while acid—oxidizing conditions are necessary to leach reduced copper species or copper sulphide minerals. Examples of the chemistry of leaching of different copper minerals are shown below.
Acid leaching reactions:    CuO(s)+H2SO4(aq)→CuSO4(aq)+H2O    CuCO3(s)+H2SO4(aq)→CuSO4(aq)+H2O+CO2(g)    CuSiO3(s)+H2SO4(aq)→CuSO4(aq)+H2O+SiO2(s)
Acid—oxidizing leaching reactions using ferric sulphate as an oxidant:    Cu2O(s)+H2SO4(aq)+Fe2(SO4)3(aq)→2CuSO4(aq)+2FeSO4(aq)+H2O    Cu2S(s)+2Fe2(SO4)3(aq)→2CuSO4(aq)+4FeSO4(aq)+S(s)    CuS(s)+Fe2 (SO4)3(aq)→CuSO4(aq)+2FeSO4(aq)+S(s)    Cu5FeS4(s)+6Fe2(SO4)3(aq)→5CuSO4(aq)+13FeSO4(aq)+4S(s)    CuFeS2(s)+2Fe2(SO4)3(aq)→CuSO4(aq)+5FeSO4(aq)+2S(s)
The rate and extent of copper leaching obtained in this series of reactions is directly linked to the availability of acid or acid and ferric ion.
Acid and ferric ion can also be consumed by “side” reactions with barren (of copper) minerals such as carbonates or other sulphides or sulphur. Ferric ion can also be consumed by hydrolysis to form precipitates such as ferric hydroxide, goethite, hematite and jarosite.    CaCO3(s)+H2SO4(aq)+H2O→CaSO4.2H2O(s)+CO2(g)    MgCO3(s)+H2SO4(aq)→MgSO4(aq)+H2O+CO2(g)    S(s)+3Fe2(SO4)3(aq)+4H2O →4H2SO4(aq)+6FeSO4(aq)    FeS2(s)+Fe2(SO4)3(aq)→3FeSO4(aq)+2S(s)    Fe2(sO4)3(aq)+6H2O→2Fe(OH)3(s)+3H2SO4(aq)    Fe2(SO4)3(aq)+4H2O→2FeO(OH)(s)+3H2SO4(aq)    Fe2(sO4)3(aq)+3H2O→Fe2O3(s)+3H2SO4(aq)    3Fe2(SO4)3(aq)+14H2O→2H3OFe3(SO4)2(OH)6(S)+5H2SO4(aq)
Note that the interaction of these reactions can be complex with some reactions consuming acid and some producing acid.
In summary, in industrial leaching operations for copper, acid and ferric ion are required in sufficient amounts to complete the leaching reactions to maximize the extraction of copper.
In many leaching operations, acid is purchased and added as a reagent to the leach solution. Generally, ferric ion is produced in situ in leaching systems, for example, by oxidation of ferrous sulphate to ferric sulphate. In lower temperature systems, the oxidation of ferrous to ferric is catalyzed by natural bacterial action in the presence of oxygen from air. At higher temperature, superatmospheric-pressure oxygen is often supplied to accelerate the rate of oxidation in an autoclave.4FeSO4+2H2SO4+O2→2Fe2(SO4)3+2H2O
An example of such a process is described in International Patent Publication WO 00/06784. This document describes a process suitable for a high-pyrites content ore. High pyrites-content ores have sulphur content sufficient to regenerate acid in situ, making this process suitable for these types of ores. In many locations where copper is leached, the cost to purchase and transport acid for leaching of copper is prohibitively expensive, where the ore type and grade is insufficient for maintenance of sulphuric acid levels in situ, that is, without making up the acid levels with exogenous sulphuric acid.
A typical process utilizing an autoclave leach at elevated temperature with superatmospheric oxygen partial pressure is described in U.S. Pat. No. 5,698,170 (King, assigned to Placer Dome, Inc.). Again this process assumes that there is sufficient sulphur present to provide acid generation, and actually produces excess acid which ends up in the raffinate stream from solvent extraction. This raffinate must be neutralized, either by addition of base or by heap leach neutralization of basic rock or ore, with or without solvent extraction of neutralized raffinate to reduce the copper concentration. The neutralized raffinate then recycles to the process providing both temperature control and acid dilution of the acidic copper solution exiting the autoclave. Ferric oxidation is not a feature of this process in which the iron species are precipitated by chemistry typified by:4CuFeS2+17O2=4H2O→4CuSO4+4H2SO4+2Fe2O3↓
Hitherto, it has not been possible to conveniently make rich ferric sulphate solutions for copper leaching at the same time as acid is formed in the autoclave, for low pyrites ores.
The reason for this is that the production of rich ferric sulphate and sulphuric acid is favored by higher autoclave temperatures and oxygen pressures. However, these same conditions favor side reactions that generate precipitated species in lieu of acid, or consume acid, resulting in the requirement for acid make-up. Also, in the case of high-pyrites containing ores, side reactions result in the precipitation of elemental sulphur, which creates processing difficulties at elevated temperatures at which viscous allotropes of sulphur form.
During autoclave pressure oxidation leaching of copper and iron sulphides and sulphur, a number of reactions will occur. These can be classified as oxidation reactions and precipitation reactions. For example, at high temperature (+150° C.):
Oxidation (all sulphide sulphur oxidizes to sulphate)    CuFeS2+4.25O2+0.5H2SO4→CuSO4+0.5Fe2(SO4)3+0.5H2O    CuS+2O2→CuSO4     FeS2+3.75O2+0.5H2O→0.5Fe2(SO4)3+0.5H2SO4     S+1.5O2+H2O→H2SO4 
Iron Precipitation Reactions    Fe2(SO4)3+3H2O→Fe2O3+3H2SO4     Fe2(sO4)3+2H2O→2Fe(OH)SO4+H2SO4 
The oxidation reactions increase the solution content of dissolved copper, ferric ion and acid. The precipitation reactions precipitate iron as either hematite or basic ferric sulphate. In the first precipitation reaction, all sulphate stays in solution and acid is produced in significant amounts (three acid formed for each ferric sulphate). The second precipitation reaction forms basic ferric sulphate. In this reaction, the formation of acid is severely curtailed as the basic ferric sulphate contains two of the three available sulphates.
The formation of hematite as the iron precipitation product results in a high strength acid solution from the autoclave process. The formation of basic ferric sulphate has generally been viewed as undesirable as the basic ferric sulphate dramatically reduces the strength of acid and ferric sulphate in the autoclave discharge solution. From an environmental perspective, basic ferric sulphates are undesirable as basic ferric sulphates will gradually decompose in tailings impoundments, resulting in slow release of acid and ferric ion. This decomposition of basic ferric sulphate can acidify a tailing and result in acid mobilization of any contaminants in the tailings solids.    Fe(OH)SO4(s)+2H2O→Fe(OH)3(s)+H2SO4(aq)    3Fe(OH)SO4(s)→Fe(OH)3(s)+Fe2(SO4)3(aq)
Unfortunately, the formation of basic ferric sulphates is favoured by (1) higher temperature and (2) increasing concentration of dissolved salts. For example, as the magnesium sulphate level is increased in solution, the “break” point indicating the onset of basic ferric sulphate precipitation advances to lower free acid concentrations.
To summarize, under certain operating conditions for pressure oxidation of copper/iron/sulphur containing ores, concentrates or residues, the autoclave leach solution will contain dissolved copper and ferric sulphate salts and sulphuric acid while the residues will contain hematite and basic ferric sulphate. The presence of basic ferric sulphate reduces (1) the available acid in the autoclave solution (acid formation by iron precipitation is attenuated) and (2) the available ferric sulphate in the autoclave solution. In addition, the presence of basic ferric sulphate will render the autoclave residue environmentally unstable. For these reasons, to avoid basic ferric sulphate formation, autoclave conditions are controlled by (1) lowering the operating temperature, (2) reducing the pulp density (solid to liquid ratio) within the limits of an overall heat balance and (3) leaching in water rather than leaching in available sulphate containing solutions. All of these control strategies are undesirable as they result in increased costs or processing complexity. For example, at lower temperature, all oxidation reactions are slower and therefore a longer autoclave oxidation time is required. This would necessarily require a larger autoclave for treatment at the lower temperature. Similarly the reduction of pulp density results in movement of more water and less solid through the autoclave, again increasing the size of the autoclave. Finally, leaching in water rather than leaching in available sulphate containing solutions may unreasonably constrain the operation of a commercial autoclave facility by disrupting the overall site “water balance”.
It is one purpose of embodiments of the present invention to provide a ready source of both sulphuric acid and ferric ion from the autoclave oxidation of copper/iron/sulphur containing feed material. It is a purpose of certain embodiments of the present invention to provide a source of ferric ion for the production of a strong oxidizing solution suitable for oxidizing other minerals such as zinc sulphides, uranium oxides, nickel and cobalt sulphides, and many others.